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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions, Bcf in Billions			9 Months Ended
	Dec. 31, 2015 USD ($) Bcf	Sep. 30, 2015 USD ($) Bcf	Sep. 30, 2016 USD ($) Subsidiary mi Bcf
Significant Accounting Policies [Line Items]
Length Of Pipeline | mi			86
Cost of Removal Obligations	$ 70.1	$ 69.9	$ 77.4
Regulatory Assets	$ 126.4	$ 125.4	$ 124.5
Number of Natural Gas Storage Outstanding | Bcf	2.5	3.7	2.9
Investments in trading securities	$ 0.7	$ 0.7	$ 1.9
Reduction of other assets	(2.9)	(3.0)
Reduction of long-term debt	(2.9)	(3.0)
Maximum
Significant Accounting Policies [Line Items]
Cash equivalents maturity period			3 months
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	2.3	2.9	$ 3.5
Not Designated as Hedging Instruments
Significant Accounting Policies [Line Items]
Derivative Assets, not designated as hedging instruments		0.1	0.2
Not Designated as Hedging Instruments | Maximum
Significant Accounting Policies [Line Items]
Derivative Assets, not designated as hedging instruments	0.1
Deferred Storm Charges
Significant Accounting Policies [Line Items]
Regulatory Assets	$ 15.4	$ 16.9	11.0
Deferred Storm Charges | Recovered over the next two and a half years
Significant Accounting Policies [Line Items]
Regulatory Assets			4.1
Environmental Costs Rate Case Costs And Other Expenditures | Recovered over the next seven and a half years
Significant Accounting Policies [Line Items]
Regulatory Assets			$ 8.4
Unitil Service; Unitil Realty; and Unitil Resources
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Subsidiary			3
Utilities
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Subsidiary			3

X

- Definition

Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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Cash Deposits

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Increase (Decrease) in Long Term Debt

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Length of natural gas transmission pipeline.

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Amount of natural gas purchase contracts outstanding under hedging program.

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Number of Subsidiaries.

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